Polycrystalline molded bodies on a base of aluminum nitride which can be produced from AlN-containing starting powders by sintering with or without applying pressure, have been known for a long time. Because of their properties such as good resistance to high temperatures and oxidation, high heat conductivity and low electric conductivity, they are acquiring increased importance, particularly as crucible material for drawing monocrystals or as substrate materials.
Due to the limited molding possibilities in conventional hot presses, sintering processes are preferred for the production of parts having complicated shapes. In such processes the starting powders must first be compacted forming green bodies of the desired final shape before they can be sintered by heating.
Among the known molding processes for producing green bodies in the form, for instance, of thin plates or thin-walled crucibles, slip casting is particularly adequate since it not only saves time and expenses but has also proved satisfactory for other special ceramic materials (advanced ceramics) like those having a base of Si.sub.3 N.sub.4 and of SiC (see E.M. Rabinovich et al., J. Mater. Sci. 1982, 17 (2), pp. 323-328; ref. in C.A. Vol. 96, 1982, No. 204230s and M. Persson et al., Mater. Sci. Monogr. 1983, 16 (Ceram. Powders) pp. 735-742; ref. in C.A. vol. 99, 1983, No. 75586y).
There is reference to a test using slip casting also for ceramic materials in British Pat. No. A-1052590. Here, organic solvents were used as liquid dispersion media exclusively. Such organic solvents include aliphatic and aromatic hydrocarbons or alcohol in which the polymer cyclopentadiene used as deflocculant is sufficiently soluble. However, in the examples sintering densities were not given for the use of ceramic powders on an AlN base with xylene as dispersion medium and subsequent sintering. Moreover, it is expressly pointed out that when using water as the dispersion medium it was practically impossible to obtain molded bodies with satisfactory qualities due to the fact that most ceramic materials contain impurities that react with water forming byproducts which unfavorably affect the physical consistency of the resulting bodies.
Another reference to the possible slip casting of ceramic powders on an AlN base is U.S. Pat. No. 4,184,884, but the indications are confined to saying that a slip casting composition with a content of approximately 50 to 70% solids in water is advantageous.
In EP-A-133275 which corresponds to U.S. Pat. No. 4,478,785 and which is concerned with the pressureless sintering of pure AlN molded bodies, slip casting is also mentioned among other known molding processes but it is indicated that such method has found practically no application for molding AlN-containing bodies. Slip casting compositions of that kind with organic solvents as dispersion medium are hard to manipulate and require additional adjuvants that must each be precisely adapted to the solvent used in order to balance the tendency to sedimentation of the AlN powder. Slip casting compositions having water as dispersion medium are not stable due to the known tendency to hydrolyzation of the aluminum nitride.
The problem that presented itself was to provide a slip casting composition with a high solids content on a base of finely divided AlN-containing powders in a liquid dispersion medium and added substances for regulating the viscosity required for slip casting, which composition would be easy to manipulate, require no additional adjuvants and be sufficiently stable both chemically and physically.